The present invention relates to devices for countering vibration in structural members, such as those of an aircraft fuselage; and particularly to such devices which can be dynamically tuned to adapt performance to changes in vibration frequency.
Aircraft engines can induce significant vibration into the fuselage. In propeller powered aircraft, the propeller blades produce air pressure pulses which strike external surfaces of the airframe thereby causing a time periodic vibration of the airframe, at about 100 Hz for example. Jet engines also produce vibration in supporting members. If left unchecked, the induced vibrations create objectionable noise in the aircraft cabin, and may result in serious fatigue of the airframe.
As a consequence vibration absorbers are attached to structural members throughout the aircraft. For example, the Fokker 50 turbo prop airplane carries 150 frame-mounted absorbers. These devices typically are a simple mass-spring system in which a mass is attached to the airframe by a resilient member that acts as a spring. Elastomeric pads and metal cantilevers have been employed as the spring. The mass-spring system is fixedly tuned to resonate at the frequency of common vibrations in the structural member of the airframe to which the absorber is attached and thus optimally absorbs the vibration energy at that frequency. The absorber has a large mechanical impedance at resonance which is proportional to the quality factor Q. Absorption (mechanical impedance) at other frequencies diminishes as a function of the deviation from the absorber's resonant frequency.
A drawback of fixedly tuned absorbers is that the frequency of airframe vibrations varies with engine speed. Engine speed varies during operation, especially in the case with jet engines. Although the absorber may be tuned to the vibration frequency (e.g. 100 Hz) which occurs at the nominal cruising speed of the aircraft, less that optimal vibration absorption occurs at other engine speeds. In addition, the tuning of elastomeric type absorbers changes with the age of the elastomeric material, and both elastomeric and metal-type absorbers change their tuning with changes in temperature.
Therefore, it is desirable to provide an absorption system which is dynamically adaptive to variation of the vibration frequency. Such a dynamic system would require a controller that senses the vibration to be absorbed and produces a control signal to alter the tuning of a mechanical absorber.